As a laser source having broad application prospects, the fiber laser has advantages of a tunable bandwidth, a higher signal-to-noise ratio, and a narrower output laser linewidth, and can be widely used in fields such as fiber sensing, fiber communication and optical processing. The fiber laser comprises three parts of a pumping source, a resonator cavity and a gain medium. Longer the length of the cavity of the fiber laser, the nonlinear effect of the fiber laser is much substantial, thus it is necessary to shorten the length of the fiber. Meanwhile, the short cavity is an important condition to achieve single longitudinal mode operation for fiber laser. Short cavity fiber laser has a simple structure and is easy to be implemented. The short cavity fiber laser is typically consists of a pair of fiber gratings, and a gain medium connected therebetween, and this structure is called Distribute Bragg Reflection (DBR) type fiber laser. The short cavity fiber laser is usually used to generate narrow linewidth laser output. A U.S company NP Photonics utilized the 2 cm long erbium-doped phosphate glass fiber (DBR) laser to obtain the laser output with the power of 100 mW and the linewidth of 2 kHz. In 1992, Ball and others achieved a 1548 nm signal frequency output with the linewidth of 47 kHz which is consistent with Bragg wavelength using a 980 nm LD pump source by adding two Bragg gratings in the two ends of the 50 cm long Er3+ doped fiber for the first time, which the two bragg gratings are 1.25 cm long with the same bragg wavelength, and reflectance of 72% and 80% respectively. In 2007, A-FR company developed a type of fiber laser with the cavity length less than 5 cm, linewidth less than 3 kHz and output power up to 150 mW. The short cavity fiber laser has several advantages such as a few numbers of longitudinal mode output, and stable output with no mode-hopping phenomena, and it is often used in field of fiber sensing. Therefore, there are important theoretical significance and application value to design a sensing system based on longitudinal mode output by the short cavity fiber laser.
In accordance with the physics definition, when an object or material is deformed due to an external force applied thereon, an interact internal force will be produced among different parts of the material in order to counteract the effect of the external force, and this internal force will try to make the material restore to the previous position prior to the deformation. Stress is defined as the internal force per unit area at a certain point on a cross section of the material. Stress will increase with the increase of the external force. With regard to a certain kind of material, there is a limit for the stress to increase. The material will be destroyed when the stress is beyond this limit. The stress at the point of this limit is defined as the ultimate stress for the material. In order to use the material safely, the stress should be lower than the ultimate stress, otherwise the material will be destroyed. Therefore, the measurement of material stress is a very important physical indicator when the material is applied in engineering. The commonly used method for strain measuring is strain electrical measuring method, which is an experimental stress analysis method. According to the relationship between stress and strain, the stress state on the surface of the material will be determined based the strain on the surface of the material measured by a resistance strain gage. However, the accuracy of this method for measuring the strain is not high, so it cannot satisfy the needs for high precision in some applications.
Therefore, a method and system for accurately measuring the deformation of the material is needed using the features of the short cavity fiber laser, which method and system may utilize the longitudinal mode output by the fiber laser as a sensing system.